ELEMENTARY CHEMICAL MICROSCOPY 



iris diaphragm D. The stand and radiant are placed at such 

 distances from L as to project a small beam of approximately 

 parallel light upon L. The milled head a serves as a fine adjust- 

 ment up and down of the lens and diaphragm. When either 

 daylight illumination, direct sunlight, or 

 a radiant at a distance are to be used, 

 the mirrors R 2 and RI are brought into 

 service, the light from the chosen source 

 being received upon R 2 , reflected upon RI, 

 and thence through the lens and dia- 

 phragm opening. When a radiant close 

 to A is used the mirror RI is raised until 

 it stands in a vertical position, thus giv- 

 ing an unobstructed passage through the 

 center of A. 



Correct illumination of the surface of 

 an object m is obtained as described 

 above by trying the lens L at different 

 distances from P and by tipping P or k 



FIG. al^ndensing L 7ns until the most satisfactory angle of in- 

 and Iris Diaphragm for clination is obtained. It may also be 

 Use with Leitz Vertical necessary to slide S slightly to the right 



Illuminator. Qr Ht Q ^ indicator t It is usua lly 



best to start with a diaphragm opening yielding a beam of light 

 which will not more than half fill the aperture of the lens L. 



Tassin Vertical Illuminator. One of the greatest annoyances 

 encountered in the work with ordinary vertical illuminators is 

 the necessity of readjusting the height of the radiant whenever a 

 change of objective is made or objects of different thicknesses are 

 studied, since refocusing is essential and this necessarily alters 

 the position of the disk or prism with reference to the axis of the 

 radiant. To obviate this defect Tassin has devised an appara- 

 tus in which the radiant either a small tungsten lamp or an 

 acetylene burner is attached to the illuminator mounting and 

 hence in focusing, both radiant and illuminator are displaced 

 simultaneously an equal amount; thus no realignment is neces- 

 sary. The construction of this device will readily be understood 



